Floating transportation vessels, such as liquefied gas carriers and barges, are capable of transporting a variety of cargoes in the liquefied state. In the present context, these liquefied cargoes have boiling points of greater than −110° C. when measured at 1 atmosphere and include liquefied petroleum gas, liquefied petrochemical gasses such as propylene and ethylene and liquefied ammonia. Liquefied petroleum gas is a useful fuel source, such as for heating appliances and vehicles, as well as being a source of hydrocarbon compounds. LPG comprises one or more of propane, n-butane and i-butane, and optionally one or more other hydrocarbons such as propylene, butylenes and ethane.
Petroleum gases can be extracted from natural gas or produced in the refining of crude oil. As a consequence, petroleum gasses normally comprise a plurality of components. It is often desirable to liquefy petroleum gases in a liquefaction facility at or near their source. As an example, petroleum gases can be stored and transported over long distances more readily as a liquid than in gaseous form because they occupy a smaller volume and may not need to be stored at high pressures. Such LPG can be stored at atmospheric pressure if maintained at or below its boiling temperature, such as at −42° C. or below, being the boiling point of the propane component. Alternatively, LPG may be stored at higher temperatures if it is pressurized above atmospheric pressure.
Petrochemical gases such as ethylene and propylene may be present in, or can be synthesized from, petroleum gas or other hydrocarbons. It is often desirable to liquefy petrochemical gases in a liquefaction facility at or near their place of separation or manufacture for similar reasons to the petroleum gases. Liquefied petrochemical gases can be stored at atmospheric pressure if maintained at or below their boiling temperature, such as at −104° C. or below, for ethylene. Alternatively, liquefied petrochemical gases may be stored at higher temperatures if they are pressurized above atmospheric pressure.
The long distance transportation of LPG or other liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere may be carried out in a suitable liquefied gas carrier, particularly an LPG carrier, such as an ocean-going tanker having one or more storage tanks to hold the liquefied cargo. These storage tanks may be insulated and/or pressurized tanks. During the loading of the tanks and the storage of liquefied cargo such as LPG in the tanks, gas, such as petroleum gas, may be produced due to the evaporation of the cargo. This evaporated cargo gas is known as boil off gas (BOG). In order to prevent the build up of BOG in the tank, a system may be provided on the carrier to re-liquefy the BOG so that it can be returned to the storage tank in a condensed state. This can be achieved by the compression and cooling of the BOG. In many systems, the compressed BOG is cooled and condensed against seawater.
There are many considerations associated with providing systems to re-liquefy boil off gas from such liquefied cargoes in floating transportation vessels. The size of the vessel imposes limitations on the space available for the re-liquefaction system. This can restrict the number and size of the compressor trains. Furthermore, size restrictions may also preclude the use of a closed refrigeration system to cool the condenser for the compressed BOG stream, such that the cooling duty may be supplied by seawater. When seawater is used, the re-liquefaction system is generally designed to operate with seawater temperatures at up to 32° C.
Liquefied cargoes such as those comprising primarily propane, particularly commercial grade propane, may further comprise relatively high concentrations of lighter components, such as ethane. It may not be possible to re-liquefy all the components of the boil off gas from such liquefied cargoes, particularly those comprising lighter components, such as ethane, present in concentrations above 3.5 mol %. Such non-condensed components may then either be returned to the liquefied cargo storage tanks in the gaseous phase, and will build up in the boil off gas in a closed system thereby increasing in concentration over time, or may be vented from the vessel in order to prevent their build up in the boil off gas. The build up or venting of non-condensed cargo components should be avoided. For instance, as the concentration of non-condensed components in the boil off gas increases, the volume of boil off gas which cannot be re-condensed will increase, reducing the effective capacity of the re-liquefaction system. The venting of non-condensed components, which may be greenhouse gases, is both environmentally and commercially undesirable.
Liquefied cargoes comprising lower boiling point components, such as those with boiling points in the range of from greater than −110° C. to −55° C. when measured at 1 atmosphere, such as the petroleum gas ethane, which may be present as a component in natural gas liquids (NGLs), and the petrochemical gas ethylene, pose particular re-liquefaction problems. For instance, seawater may be unable to provide sufficient cooling duty to re-liquefy the ethane or ethylene component of BOG. In addition the re-liquefaction of such BOG components may require greater compression (e.g. compared to the re-liquefaction of higher boiling point components such as propane).
Typically the re-liquefaction of ethylene requires a compression system capable of compressing the ethylene BOG to a pressure of approximately 51 bar, such as a compression system comprising three or more stages, and a cooling medium at a temperature of 9.5° C. or below in order to condense the compressed BOG stream.
A need exists to provide an improved method of cooling, particularly re-liquefying, boil off gas from a liquefied cargo having a boiling point of greater than −110° C. when measured at 1 atmosphere and comprising a plurality of components in a floating transportation vessel. In particular, a method which provides improved cooling, particularly re-liquefaction, of lighter components of the cargo is desirable.